1. Field of the Invention
The present invention relates to an acrylic rubber composition, and more particularly to an acrylic rubber composition capable of giving vulcanization products having an improved compression set.
2. Related Prior Art
Known typical vulcanizable, halogen-containing acrylic rubber systems are those based on polyamine or its carbamate. The resulting vulcanization products still have such disadvantages as a poor compression set and a poor resistance to heat ageing. To improve these disadvantages, it was proposed to copolymerize acrylic rubber with a cross-linking site monomer containing an active halogen such as vinyl chloroacetate, etc. (JP-B-39-20515).
Such known active halogen-containing acrylic rubber vulcanizable systems contain a vulcanizer system of fatty acid alkali metal salt/sulfur or a vulcanizer system of trithiocyanuric acid/alkyldithiocarbamic acid metal salt. The former vulcanizer system can give vulcanization products having good normal state physical properties and a good resistance to heat ageing, but a poor compression set. Thus, the latter vulcanizer system capable of giving vulcanization products having a good compression set has been practically used up to now as a leading vulcanizer system for use in the vulcanization molding of O-rings, gaskets, etc. requiring a good compression set.
However, the vulcanizer system of trithiocyanuric acid/alkyldithiocarbamic acid metal salt still has the following problems, one of which is generation of unpleasant odor from alkyldithiocarbamic acid metal salt during the vulcanization. Furthermore, in the rubber molding and processing fields, a shift from the now leading press molding to the injection molding is under way to increase the productivity, but this vulcanizer system has a problem of poor injection moldability.
To improve the poor compression set of the vulcanizer system of fatty acid alkali metal salt/sulfur capable of giving vulcanization products having good normal state physical properties, a good resistance to heat ageing and having a good injection moldability without generation of unpleasant odor, on the other hand, it was proposed to incorporate an N-substituted bismaleimide into the vulcanizer system (JP-B-48-24019). The proposed vulcanizer system has an improved compression set indeed, but has too high a vulcanization rate and a cross-linking density, resulting in deterioration of breaking strength, (stress strength at break) breaking elongation, (elongation at break) etc.
It was also proposed to add an unsaturated monocarboxylic acid ester such as acrylic acid esters to the vulcanizer system of fatty acid alkali metal salt/sulfur (JP-A-3-31351). However, the acrylic acid esters have problems not only of generation of unpleasant odor and skin irritation during the kneading or vulcanization, but also of considerably retarded vulcanization and lowered modulus or breaking strength, and derioration of mold releasability due to the retarded vulcanization.
Furthermore, it was proposed to add a thioether compound to the vulcanizer system (JP-A-4-142356). Generally, the resistance to heat ageing of acrylic rubber is connected to a brittling tendency and only available effective antioxidants for relieving the brittling tendency are specific amine-based antioxidants. Thus, even use of a thioether compound in place of the amine-based antioxidant inevitably gives vulcanization products having a poor resistance to heat ageing.